Check `#Source.path` contains a `shell.nix`

If shell.nix does not exists trackbone exit with a strange error:

[2020-10-27 08:56:11]  INFO corp/keycloak-vault-corp: Fetching git source...
Counting objects: 4810, done.
Compressing objects: 100% (2591/2591), done.
Total 4810 (delta 2589), reused 4084 (delta 2187)
[2020-10-27 08:56:12]  INFO Droppin' in nix-shell!
error: value is a function while a set was expected, at /run/user/1000/trackbone-113206238/trackbone-shell.nix:6:1

Return a better error in this case